我的公式有误吗？或者是否有任何代码行错了？

Question

这是我的完整源代码。

好像我无法获得输出。我可以成功获得用户界面，但是当我输入任何数字以获取球体的表面积和体积时，我无法获得确切的答案。我不知道是否弄乱了公式，还是事件处理代码出了点问题。

import javax.swing.*;
import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.*;
import java.text.DecimalFormat;

public class Sphere extends JFrame implements ActionListener {
    private JLabel radius, result;
    private JTextField tfRadius, tfResult;
    private JButton btnArea, btnVol; 
    private JPanel p1, p2;

    public static void main(String[] args) {
        Sphere S = new Sphere();
        S.setSize(300, 150);
        S.setVisible(true);
        S.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);        
    }

    public Sphere() {
        setTitle("Sphere");
        setLayout(new BorderLayout());

        radius = new JLabel("Radius");
        result = new JLabel("Result");
        tfRadius = new JTextField(10);
        tfResult = new JTextField(10);
        tfResult.setEditable(false);
        btnArea = new JButton("Surface Area");
        btnVol = new JButton("Volume");

        p1 = new JPanel();
        p1.setLayout(new GridLayout(2, 2, 2, 2));
        p1.add(radius);
        p1.add(tfRadius);
        p1.add(result);
        p1.add(tfResult);

        p2 = new JPanel();
        p2.add(btnArea); 
        p2.add(btnVol);

        add(p1, BorderLayout.NORTH);
        add(p2, BorderLayout.SOUTH);   

        btnArea.addActionListener(this);
        btnVol.addActionListener(this);
    }

    public void actionPerformed(ActionEvent e) {
        DecimalFormat df = new DecimalFormat("0.0000");
        double result = 0;
        //double pi = 3.142; before I edited.
        double radius = Double.parseDouble(tfRadius.getText());

        //action from the respective button
        if(e.getSource() == btnArea) {
            result = 4 * (Math.PI * Math.pow(radius, 2)); //edited from pi to Math.pi
        } else if (e.getSource() == btnVol) {
            result = 4.0/3.0 * (Math.PI * Math.pow(radius, 3)); //edited from 1.333 to 4.0/3.0
        }

        tfResult.setText(String.valueOf(df.format(result)));            
    }
}